This item is from the Yet Another Enhanced IDE/Fast-ATA/ATA-2 FAQ, by John Wehman and Peter den Haan with numerous contributions by others. (v1.92).
A rather different issue was the 504MB (or, equivalently, 528 million bytes) capacity limit that becomes apparent when accessing IDE drives through the BIOS. It is caused by the disk geometry (cylinders, heads, sectors) supported by the combination of an IDE drive and the BIOS' software interface ('int13'). Both IDE/ATA and the BIOS are capable of supporting huge disks, but their combined limitations conspire to restrict the useful capacity of the drive to 504MB. Since only MSDOS still uses the BIOS for harddisk access, this is sometimes erroneously thought to be an MSDOS limitation; other operating systems experience the same restrictions at boot time though.
An Enhanced BIOS works around this problem by representing the drive to the software using a different geometry than the native geometry of the drive itself. This juggling act is called 'translation'. For example, if your drive has 1500 cylinders and 16 heads, a translating BIOS will make software programs think that the drive has 750 cylinders and 32 heads.
You do not need an "EIDE" adapter to use harddisks greater than 504MB.
The de facto standard is described in the Microsoft/IBM "INT 13 Extensions" document. Phoenix has presented a superset in their "Phoenix Enhanced BIOS" specification. Phoenix, AMI, Award and MR BIOSes are based on the Microsoft/IBM specification.